DDoS Open Threat Signaling (DOTS) Requirements

Abstract

This document defines the requirements for the Distributed Denial-of-
Service (DDoS) Open Threat Signaling (DOTS) protocols enabling
coordinated response to DDoS attacks.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8612.

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described in the Simplified BSD License.

1. Introduction

1.1. Context and Motivation

Distributed Denial-of-Service (DDoS) attacks afflict networks
connected to the Internet, plaguing network operators at service
providers and enterprises around the world. High-volume attacks
saturating inbound links are now common as attack scale and frequency
continue to increase.

The prevalence and impact of these DDoS attacks has led to an
increased focus on coordinated attack response. However, many
enterprises lack the resources or expertise to operate on-premise
attack mitigation solutions themselves, or are constrained by local
bandwidth limitations. To address such gaps, service providers have
begun to offer on-demand traffic scrubbing services, which are
designed to separate the DDoS attack traffic from legitimate traffic
and forward only the latter.

Today, these services offer proprietary interfaces for subscribers to
request attack mitigation. Such proprietary interfaces tie a
subscriber to a service and limit the abilities of network elements
that would otherwise be capable of participating in attack
mitigation. As a result of signaling interface incompatibility,
attack responses may be fragmented or otherwise incomplete, leaving
operators in the attack path unable to assist in the defense.

A standardized method to coordinate a real-time response among
involved operators will increase the speed and effectiveness of DDoS
attack mitigation and reduce the impact of these attacks. This
document describes the required characteristics of protocols that
enable attack response coordination and mitigation of DDoS attacks.

DDoS Open Threat Signaling (DOTS) communicates the need for defensive
action in anticipation of or in response to an attack, but it does
not dictate the implementation of these actions. The DOTS use cases
are discussed in [DOTS-USE], and the DOTS architecture is discussed
in [DOTS-ARCH].

1.2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

These capitalized words are used to signify the requirements for the
DOTS protocols design.

This document adopts the following terms:

DDoS: A distributed denial-of-service attack in which traffic
originating from multiple sources is directed at a target on a
network. DDoS attacks are intended to cause a negative impact on
the availability and/or functionality of an attack target.
Denial-of-service considerations are discussed in detail in
[RFC4732].
DDoS attack target: A network-connected entity that is the target of
a DDoS attack. Potential targets include (but are not limited to)
network elements, network links, servers, and services.
DDoS attack telemetry: Collected measurements and behavioral
characteristics defining the nature of a DDoS attack.
Countermeasure: An action or set of actions focused on recognizing
and filtering out specific types of DDoS attack traffic while
passing legitimate traffic to the attack target. Distinct
countermeasures can be layered to defend against attacks combining
multiple DDoS attack types.
Mitigation: A set of countermeasures enforced against traffic
destined for the target or targets of a detected or reported DDoS
attack, where countermeasure enforcement is managed by an entity
in the network path between attack sources and the attack target.
Mitigation methodology is out of scope for this document.
Mitigator: An entity, typically a network element, capable of
performing mitigation of a detected or reported DDoS attack. The
means by which this entity performs these mitigations and how they
are requested of it are out of scope for this document. The
mitigator and DOTS server receiving a mitigation request are
assumed to belong to the same administrative entity.
DOTS client: A DOTS-aware software module responsible for requesting
attack response coordination with other DOTS-aware elements.
DOTS server: A DOTS-aware software module handling and responding to
messages from DOTS clients. The DOTS server enables mitigation on
behalf of the DOTS client, if requested, by communicating the DOTS
client's request to the mitigator and returning selected mitigator
feedback to the requesting DOTS client.
DOTS agent: Any DOTS-aware software module capable of participating
in a DOTS signal or data channel. It can be a DOTS client, DOTS
server, or, as a logical agent, a DOTS gateway.
DOTS gateway: A DOTS-aware software module resulting from the
logical concatenation of the functionality of a DOTS server and a
DOTS client into a single DOTS agent. This functionality is
analogous to a Session Initiation Protocol (SIP) [RFC3261] Back-
to-Back User Agent (B2BUA) [RFC7092]. A DOTS gateway has a
client-facing side, which behaves as a DOTS server for downstream
clients, and a server-facing side, which performs the role of a
DOTS client for upstream DOTS servers. Client-domain DOTS
gateways are DOTS gateways that are in the DOTS client's domain,
while server-domain DOTS gateways denote DOTS gateways that are in
the DOTS server's domain. A DOTS gateway may terminate multiple
discrete DOTS client connections and may aggregate these into one
or more connections. DOTS gateways are described further in
[DOTS-ARCH].
Signal channel: A bidirectional, mutually authenticated
communication channel between DOTS agents that is resilient even
in conditions leading to severe packet loss such as a volumetric
DDoS attack causing network congestion.
DOTS signal: A status/control message transmitted over the
authenticated signal channel between DOTS agents, used to indicate
the client's need for mitigation or to convey the status of any
requested mitigation.
Heartbeat: A message transmitted between DOTS agents over the signal
channel, used as a keep-alive and to measure peer health.
Data channel: A bidirectional, mutually authenticated communication
channel between two DOTS agents used for infrequent but reliable
bulk exchange of data not easily or appropriately communicated
through the signal channel. Reliable bulk data exchange may not
function well or at all during attacks causing network congestion.
The data channel is not expected to operate in such conditions.
Filter: A specification of a matching network traffic flow or set of
flows. The filter will typically have a policy associated with
it, e.g., rate-limiting or discarding matching traffic [RFC4949].
Drop-list: A list of filters indicating sources from which traffic
should be blocked regardless of traffic content.
Accept-list: A list of filters indicating sources from which traffic
should always be allowed regardless of contradictory data gleaned
in a detected attack.
Multihomed DOTS client: A DOTS client exchanging messages with
multiple DOTS servers, each in a separate administrative domain.

2. Requirements

The expected layout and interactions amongst DOTS entities is
described in the DOTS Architecture [DOTS-ARCH].

The goal of the DOTS requirements specification is to specify the
requirements for DOTS signal channel and data channel protocols that
have different application and transport-layer requirements. This
section describes the required features and characteristics of the
DOTS protocols.

The goal of DOTS protocols is to enable and manage mitigation on
behalf of a network domain or resource that is or may become the
focus of a DDoS attack. An active DDoS attack against the entity
controlling the DOTS client need not be present before establishing a
communication channel between DOTS agents. Indeed, establishing a
relationship with peer DOTS agents during normal network conditions
provides the foundation for a more rapid attack response against
future attacks, as all interactions setting up DOTS, including any
business or service-level agreements, are already complete.
Reachability information of peer DOTS agents is provisioned to a DOTS
client using a variety of manual or dynamic methods. Once a
relationship between DOTS agents is established, regular
communication between DOTS clients and servers enables a common
understanding of the DOTS agents' health and activity.

The DOTS protocol must, at a minimum, make it possible for a DOTS
client to request aid mounting a defense against a suspected attack.
This defense could be coordinated by a DOTS server and include
signaling within or between domains as requested by local operators.
DOTS clients should similarly be able to withdraw aid requests. DOTS
requires no justification from DOTS clients for requests for help,
nor do DOTS clients need to justify withdrawing help requests; the
decision is local to the DOTS clients' domain. Multihomed DOTS
clients must be able to select the appropriate DOTS server(s) to
which a mitigation request is to be sent. The method for selecting
the appropriate DOTS server in a multihomed environment is out of
scope for this document.

DOTS protocol implementations face competing operational goals when
maintaining this bidirectional communication stream. On the one
hand, DOTS must include measures to ensure message confidentiality,
integrity, authenticity, and replay protection to keep the protocols
from becoming additional vectors for the very attacks it is meant to
help fight off. On the other hand, the protocol must be resilient
under extremely hostile network conditions, providing continued
contact between DOTS agents even as attack traffic saturates the
link. Such resiliency may be developed several ways, but
characteristics such as small message size, asynchronous
notifications, redundant message delivery, and minimal connection
overhead (when possible, given local network policy) will tend to
contribute to the robustness demanded by a viable DOTS protocol.
Operators of peer DOTS-enabled domains may enable either quality-of-
service or class-of-service traffic tagging to increase the
probability of successful DOTS signal delivery, but DOTS does not
require such policies be in place and should be viable in their
absence.

The DOTS server and client must also have some standardized method of
defining the scope of any mitigation, as well as managing other
mitigation-related configurations.

Finally, DOTS should be sufficiently extensible to meet future needs
in coordinated attack defense, although this consideration is
necessarily superseded by the other operational requirements.

2.1. General Requirements

GEN-001 Extensibility: Protocols and data models developed as part
of DOTS MUST be extensible in order to keep DOTS adaptable to
proprietary DDoS defenses. Future extensions MUST be backward
compatible. Implementations of older protocol versions MUST
ignore optional information added to DOTS messages as part of
newer protocol versions. Implementations of older protocol
versions MUST reject DOTS messages carrying mandatory information
as part of newer protocol versions.
GEN-002 Resilience and Robustness: The signaling protocol MUST be
designed to maximize the probability of signal delivery even under
the severely constrained network conditions caused by attack
traffic. Additional means to enhance the resilience of DOTS
protocols, including when multiple DOTS servers are provisioned to
the DOTS clients, SHOULD be considered. The protocol MUST be
resilient, that is, continue operating despite message loss and
out-of-order or redundant message delivery. In support of
signaling protocol robustness, DOTS signals SHOULD be conveyed
over transport and application protocols not susceptible to head-
of-line blocking. These requirements are at SHOULD strength to
handle middle-boxes and firewall traversal.
GEN-003 Bulk Data Exchange: Infrequent bulk data exchange between
DOTS agents can also significantly augment attack response
coordination, permitting such tasks as population of drop- or
accept-listed source addresses, address or prefix group aliasing,
exchange of incident reports, and other hinting or configuration
supplementing attack responses.

As the resilience requirements for the DOTS signal channel mandate
a small signal message size, a separate, secure data channel
utilizing a reliable transport protocol MUST be used for bulk data
exchange. However, reliable bulk data exchange may not be
possible during attacks causing network congestion.

GEN-004 Mitigation Hinting: DOTS clients may have access to attack
details that can be used to inform mitigation techniques. Example
attack details might include locally collected fingerprints for an
on-going attack, or anticipated or active attack focal points
based on other threat intelligence. DOTS clients MAY send
mitigation hints derived from attack details to DOTS servers, with
the full understanding that the DOTS server MAY ignore mitigation
hints. Mitigation hints MUST be transmitted across the signal
channel, as the data channel may not be functional during an
attack. DOTS-server handling of mitigation hints is
implementation-specific.
GEN-005 Loop Handling: In certain scenarios, typically involving
misconfiguration of DNS or routing policy, it may be possible for
communication between DOTS agents to loop. Signal and data
channel implementations should be prepared to detect and terminate
such loops to prevent service disruption.

2.2. Signal Channel Requirements

SIG-001 Use of Common Transport Protocols: DOTS MUST operate over
common, widely deployed and standardized transport protocols.
While connectionless transport such as the User Datagram Protocol
(UDP) [RFC768] SHOULD be used for the signal channel, the
Transmission Control Protocol (TCP) [RFC793] MAY be used if
necessary due to network policy or middlebox capabilities or
configurations.
SIG-002 Sub-MTU Message Size: To avoid message fragmentation and the
consequently decreased probability of message delivery over a
congested link, signaling protocol message size MUST be kept under
the signaling Path Maximum Transmission Unit (PMTU), including the
byte overhead of any encapsulation, transport headers, and
transport- or message-level security. If the total message size
exceeds the PMTU, the DOTS agent MUST split the message into
separate messages; for example, the list of mitigation scope types
could be split into multiple lists and each list conveyed in a new
message.

DOTS agents can attempt to learn PMTU using the procedures
discussed in [IP-FRAG-FRAGILE]. If the PMTU cannot be discovered,
DOTS agents MUST assume a PMTU of 1280 bytes, as IPv6 requires
that every link in the Internet have an MTU of 1280 octets or
greater as specified in [RFC8200]. If IPv4 support on legacy or
otherwise unusual networks is a consideration and the PMTU is
unknown, DOTS implementations MAY assume a PMTU of 576 bytes for
IPv4 datagrams, as every IPv4 host must be capable of receiving a
packet whose length is equal to 576 bytes as discussed in [RFC791]
and [RFC1122].

SIG-003 Bidirectionality: To support peer health detection, to
maintain an active signal channel, and to increase the probability
of signal delivery during an attack, the signal channel MUST be
bidirectional, with client and server transmitting signals to each
other at regular intervals regardless of any client request for
mitigation. The bidirectional signal channel MUST support
unidirectional messaging to enable notifications between DOTS
agents.
SIG-004 Channel Health Monitoring: DOTS agents MUST support exchange
of heartbeat messages over the signal channel to monitor channel
health. These keep-alives serve to maintain any on-path NAT or
Firewall bindings to avoid cryptographic handshake for new
mitigation requests. The heartbeat interval during active
mitigation could be negotiable based on NAT/Firewall
characteristics. Absent information about the NAT/Firewall
characteristics, DOTS agents need to ensure its on-path NAT or
Firewall bindings do not expire, by using the keep-alive frequency
discussed in Section 3.5 of [RFC8085].

To support scenarios in which loss of heartbeat is used to trigger
mitigation, and to keep the channel active, DOTS servers MUST
solicit heartbeat exchanges after successful mutual
authentication. When DOTS agents are exchanging heartbeats and no
mitigation request is active, either agent MAY request changes to
the heartbeat rate. For example, a DOTS server might want to
reduce heartbeat frequency or cease heartbeat exchanges when an
active DOTS client has not requested mitigation, in order to
control load.

Following mutual authentication, a signal channel MUST be
considered active until a DOTS agent explicitly ends the session.
When no attack traffic is present, the signal channel MUST be
considered active until either DOTS agent fails to receive
heartbeats from the other peer after a mutually agreed upon
retransmission procedure has been exhausted. Peer DOTS agents
MUST regularly send heartbeats to each other while a mitigation
request is active. Because heartbeat loss is much more likely
during volumetric attack, DOTS agents SHOULD avoid signal channel
termination when mitigation is active and heartbeats are not
received by either DOTS agent for an extended period. The
exception circumstances to terminating the signal channel session
during active mitigation are discussed below:

To handle a possible DOTS server restart or crash, the DOTS
clients MAY attempt to establish a new signal channel session
but MUST continue to send heartbeats on the current session so
that the DOTS server knows the session is still alive. If the
new session is successfully established, the DOTS client can
terminate the current session.

DOTS servers are assumed to have the ability to monitor the
attack, using feedback from the mitigator and other available
sources, and MAY use the absence of attack traffic and lack of
client heartbeats as an indication the signal channel is
defunct.

SIG-005 Channel Redirection: In order to increase DOTS operational
flexibility and scalability, DOTS servers SHOULD be able to
redirect DOTS clients to another DOTS server at any time. DOTS
clients MUST NOT assume the redirection target DOTS server shares
security state with the redirecting DOTS server. DOTS clients are
free to attempt abbreviated security negotiation methods supported
by the protocol, such as DTLS session resumption, but MUST be
prepared to negotiate new security state with the redirection
target DOTS server. The redirection DOTS server and redirecting
DOTS server MUST belong to the same administrative domain.

Due to the increased likelihood of packet loss caused by link
congestion during an attack, DOTS servers SHOULD NOT redirect
while mitigation is enabled during an active attack against a
target in the DOTS client's domain.

SIG-006 Mitigation Requests and Status: Authorized DOTS clients MUST
be able to request scoped mitigation from DOTS servers. DOTS
servers MUST send status to the DOTS clients about mitigation
requests. If a DOTS server rejects an authorized request for
mitigation, the DOTS server MUST include a reason for the
rejection in the status message sent to the client.

DOTS servers MUST regularly send mitigation status updates to
authorized DOTS clients that have requested and been granted
mitigation. If unreliable transport is used for the signal
channel protocol, due to the higher likelihood of packet loss
during a DDoS attack, DOTS servers need to send the mitigation
status multiple times at regular intervals following the data
transmission guidelines discussed in Section 3.1.3 of [RFC8085].

When DOTS client-requested mitigation is active, DOTS server
status messages MUST include the following mitigation metrics:

Total number of packets blocked by the mitigation

Current number of packets per second blocked

Total number of bytes blocked

Current number of bytes per second blocked

DOTS clients MAY take these metrics into account when determining
whether to ask the DOTS server to cease mitigation.

A DOTS client MAY withdraw a mitigation request at any time
regardless of whether mitigation is currently active. The DOTS
server MUST immediately acknowledge a DOTS client's request to
stop mitigation.

To protect against route or DNS flapping caused by a client
rapidly toggling mitigation, and to dampen the effect of
oscillating attacks, DOTS servers MAY allow mitigation to continue
for a limited period after acknowledging a DOTS client's
withdrawal of a mitigation request. During this period, DOTS
server status messages SHOULD indicate that mitigation is active
but terminating. DOTS clients MAY reverse the mitigation
termination during this active-but-terminating period with a new
mitigation request for the same scope. The DOTS server MUST treat
this request as a mitigation lifetime extension (see SIG-007).

The initial active-but-terminating period is both implementation-
and deployment-specific, but SHOULD be sufficiently long enough to
absorb latency incurred by route propagation. If a DOTS client
refreshes the mitigation before the active-but-terminating period
elapses, the DOTS server MAY increase the active-but-terminating
period up to a maximum of 300 seconds (5 minutes). After the
active-but-terminating period elapses, the DOTS server MUST treat
the mitigation as terminated, as the DOTS client is no longer
responsible for the mitigation.

SIG-007 Mitigation Lifetime: DOTS servers MUST support mitigations
for a negotiated time interval and MUST terminate a mitigation
when the lifetime elapses. DOTS servers also MUST support renewal
of mitigation lifetimes in mitigation requests from DOTS clients,
allowing clients to extend mitigation as necessary for the
duration of an attack.

DOTS servers MUST treat a mitigation terminated due to lifetime
expiration exactly as if the DOTS client originating the
mitigation had asked to end the mitigation, including the active-
but-terminating period, as described above in SIG-005.

DOTS clients MUST include a mitigation lifetime in all mitigation
requests.

DOTS servers SHOULD support indefinite mitigation lifetimes,
enabling architectures in which the mitigator is always in the
traffic path to the resources for which the DOTS client is
requesting protection. DOTS clients MUST be prepared to not be
granted mitigations with indefinite lifetimes. DOTS servers MAY
refuse mitigations with indefinite lifetimes for policy reasons.
The reasons themselves are out of scope for this document. If the

DOTS server does not grant a mitigation request with an indefinite
mitigation lifetime, it MUST set the lifetime to a value that is
configured locally. That value MUST be returned in a reply to the
requesting DOTS client.

SIG-008 Mitigation Scope: DOTS clients MUST indicate desired
mitigation scope. The scope type will vary depending on the
resources requiring mitigation. All DOTS agent implementations
MUST support the following required scope types:

DOTS servers MUST be able to resolve domain names and (when
supported) URIs. How name resolution is managed on the DOTS
server is implementation-specific.

DOTS agents MUST support mitigation scope aliases, allowing DOTS
clients and servers to refer to collections of protected resources
by an opaque identifier created through the data channel, direct
configuration, or other means. Domain name and URI mitigation
scopes may be thought of as a form of scope alias in which the
addresses to which the domain name or URI resolve represent the
full scope of the mitigation.

If there is additional information available narrowing the scope
of any requested attack response, such as targeted port range,
protocol, or service, DOTS clients SHOULD include that information
in client mitigation requests. DOTS clients MAY also include
additional attack details. DOTS servers MAY ignore such
supplemental information when enabling countermeasures on the
mitigator.

As an active attack evolves, DOTS clients MUST be able to adjust
as necessary the scope of requested mitigation by refining the
scope of resources requiring mitigation.

A DOTS client may obtain the mitigation scope through direct
provisioning or through implementation-specific methods of
discovery. DOTS clients MUST support at least one mechanism to
obtain mitigation scope.

SIG-009 Mitigation Efficacy: When a mitigation request is active,
DOTS clients MUST be able to transmit a metric of perceived
mitigation efficacy to the DOTS server. DOTS servers MAY use the
efficacy metric to adjust countermeasures activated on a mitigator
on behalf of a DOTS client.
SIG-010 Conflict Detection and Notification: Multiple DOTS clients
controlled by a single administrative entity may send conflicting
mitigation requests as a result of misconfiguration, operator
error, or compromised DOTS clients. DOTS servers in the same
administrative domain attempting to honor conflicting requests may
flap network route or DNS information, degrading the networks
attempting to participate in attack response with the DOTS
clients. DOTS servers in a single administrative domain SHALL
detect such conflicting requests and SHALL notify the DOTS clients
in conflict. The notification MUST indicate the nature and scope
of the conflict, for example, the overlapping prefix range in a
conflicting mitigation request.
SIG-011 Network Address Translator Traversal: DOTS clients may be
deployed behind a Network Address Translator (NAT) and need to
communicate with DOTS servers through the NAT. DOTS protocols
MUST therefore be capable of traversing NATs.

If UDP is used as the transport for the DOTS signal channel, all
considerations in "Middlebox Traversal Guidelines" in [RFC8085]
apply to DOTS. Regardless of transport, DOTS protocols MUST
follow established best common practices established in BCP 127
for NAT traversal [RFC4787] [RFC6888] [RFC7857].

2.3. Data Channel Requirements

The data channel is intended to be used for bulk data exchanges
between DOTS agents. Unlike the signal channel, the data channel is
not expected to be constructed to deal with attack conditions. As
the primary function of the data channel is data exchange, a reliable
transport is required in order for DOTS agents to detect data
delivery success or failure.

The data channel provides a protocol for DOTS configuration and
management. For example, a DOTS client may submit to a DOTS server a
collection of prefixes it wants to refer to by alias when requesting
mitigation, to which the server would respond with a success status
and the new prefix group alias, or an error status and message in the
event the DOTS client's data channel request failed.

DATA-001 Reliable transport: Messages sent over the data channel
MUST be delivered reliably in the order sent.
DATA-003 Resource Configuration: To help meet the general and signal
channel requirements in Sections 2.1 and 2.2, DOTS server
implementations MUST provide an interface to configure resource
identifiers, as described in SIG-008. DOTS server implementations
MAY expose additional configurability. Additional configurability
is implementation-specific.
DATA-004 Policy Management: DOTS servers MUST provide methods for
DOTS clients to manage drop- and accept-lists of traffic destined
for resources belonging to a client.

For example, a DOTS client should be able to create a drop- or
accept-list entry, retrieve a list of current entries from either
list, update the content of either list, and delete entries as
necessary.

How a DOTS server authorizes DOTS client management of drop- and
accept-list entries is implementation-specific.

2.4. Security Requirements

DOTS must operate within a particularly strict security context, as
an insufficiently protected signal or data channel may be subject to
abuse, enabling or supplementing the very attacks DOTS purports to
mitigate.

SEC-001 Peer Mutual Authentication: DOTS agents MUST authenticate
each other before a DOTS signal or data channel is considered
valid. The method of authentication is not specified in this
document but should follow current IETF best practices [RFC7525]
with respect to any cryptographic mechanisms to authenticate the
remote peer.
SEC-002 Message Confidentiality, Integrity, and Authenticity: DOTS
protocols MUST take steps to protect the confidentiality,
integrity, and authenticity of messages sent between client and
server. While specific transport- and message-level security
options are not specified, the protocols MUST follow current IETF
best practices [RFC7525] for encryption and message
authentication. Client-domain DOTS gateways are more trusted than
DOTS clients, while server-domain DOTS gateways and DOTS servers
share the same level of trust. A security mechanism at the
transport layer (TLS or DTLS) is thus adequate to provide security
between peer DOTS agents.

In order for DOTS protocols to remain secure despite advancements
in cryptanalysis and traffic analysis, DOTS agents MUST support
secure negotiation of the terms and mechanisms of protocol
security, subject to the interoperability and signal message size
requirements in Section 2.2.

While the interfaces between downstream DOTS server and upstream
DOTS client within a DOTS gateway are implementation-specific,
those interfaces nevertheless MUST provide security equivalent to
that of the signal channels bridged by gateways in the signaling
path. For example, when a DOTS gateway consisting of a DOTS
server and DOTS client is running on the same logical device, the
two DOTS agents could be implemented within the same process
security boundary.

SEC-003 Data Privacy and Integrity: Transmissions over the DOTS
protocols are likely to contain operationally or privacy-sensitive
information or instructions from the remote DOTS agent. Theft,
modification, or replay of message transmissions could lead to
information leaks or malicious transactions on behalf of the
sending agent (see Section 4). Consequently, data sent over the
DOTS protocols MUST be encrypted using secure transports (TLS or
DTLS). DOTS servers MUST enable means to prevent leaking
operationally or privacy-sensitive data. Although administrative
entities participating in DOTS may detail what data may be
revealed to third-party DOTS agents, such considerations are not
in scope for this document.
SEC-004 Message Replay Protection: To prevent a passive attacker
from capturing and replaying old messages, and thereby potentially
disrupting or influencing the network policy of the receiving DOTS
agent's domain, DOTS protocols MUST provide a method for replay
detection and prevention.

Within the signal channel, messages MUST be uniquely identified
such that replayed or duplicated messages can be detected and
discarded. Unique mitigation requests MUST be processed at most
once.

SEC-005 Authorization: DOTS servers MUST authorize all messages from
DOTS clients that pertain to mitigation, configuration, filtering,
or status.

DOTS servers MUST reject mitigation requests with scopes that the
DOTS client is not authorized to manage.

Likewise, DOTS servers MUST refuse to allow creation,
modification, or deletion of scope aliases and drop- or accept-
lists when the DOTS client is unauthorized.

The modes of authorization are implementation-specific.

2.5. Data Model Requirements

A well-structured DOTS data model is critical to the development of
successful DOTS protocols.

DM-001 Structure: The data-model structure for the DOTS protocol MAY
be described by a single module or be divided into related
collections of hierarchical modules and submodules. If the data
model structure is split across modules, those distinct modules
MUST allow references to describe the overall data model's
structural dependencies.
DM-002 Versioning: To ensure interoperability between DOTS protocol
implementations, data models MUST be versioned. How the protocols
represent data-model versions is not defined in this document.
DM-003 Mitigation Status Representation: The data model MUST provide
the ability to represent a request for mitigation and the
withdrawal of such a request. The data model MUST also support a
representation of currently-requested mitigation status, including
failures and their causes.
DM-004 Mitigation Scope Representation: The data model MUST support
representation of a requested mitigation's scope. As mitigation
scope may be represented in several different ways, per SIG-008,
the data model MUST include extensible representation of
mitigation scope.
DM-005 Mitigation Lifetime Representation: The data model MUST
support representation of a mitigation request's lifetime,
including mitigations with no specified end time.
DM-006 Mitigation Efficacy Representation: The data model MUST
support representation of a DOTS client's understanding of the
efficacy of a mitigation enabled through a mitigation request.
DM-007 Acceptable Signal Loss Representation: The data model MUST be
able to represent the DOTS agent's preference for acceptable
signal loss when establishing a signal channel. Measurements of
loss might include, but are not restricted to, number of
consecutive missed heartbeat messages, retransmission count, or
request timeouts.
DM-008 Heartbeat Interval Representation: The data model MUST be
able to represent the DOTS agent's preferred heartbeat interval,
which the client may include when establishing the signal channel,
as described in SIG-003.
DM-009 Relationship to Transport: The DOTS data model MUST NOT make
any assumptions about specific characteristics of any given
transport into the data model, but instead represent the fields in
the model explicitly.

3. Congestion Control Considerations

3.1. Signal Channel

As part of a protocol expected to operate over links affected by DDoS
attack traffic, the DOTS signal channel MUST NOT contribute
significantly to link congestion. To meet the signal channel
requirements above, DOTS signal channel implementations SHOULD
support connectionless transports. However, some connectionless
transports, when deployed naively, can be a source of network
congestion, as discussed in [RFC8085]. Signal channel
implementations using such connectionless transports, such as UDP,
therefore MUST include a congestion control mechanism.

3.2. Data Channel

As specified in DATA-001, the data channel requires reliable, in-
order message delivery. Data channel implementations using an IETF
standard congestion-controlled transport protocol may rely on the
transport implementation's built-in congestion control mechanisms.

4. Security Considerations

This document informs future protocols under development and so does
not have security considerations of its own. However, operators
should be aware of potential risks involved in deploying DOTS. DOTS
agent impersonation and signal blocking are discussed here.
Additional DOTS security considerations may be found in [DOTS-ARCH]
and DOTS protocol documents.

Impersonation of either a DOTS server or a DOTS client could have
catastrophic impact on operations in either domain. If an attacker
has the ability to impersonate a DOTS client, that attacker can
affect policy on the network path to the DOTS client's domain up to
and including instantiation of drop-lists blocking all inbound
traffic to networks for which the DOTS client is authorized to
request mitigation.

Similarly, an impersonated DOTS server may be able to act as a sort
of malicious DOTS gateway, intercepting requests from the downstream
DOTS client and modifying them before transmission to the DOTS server
to inflict the desired impact on traffic to or from the DOTS client's
domain. Among other things, this malicious DOTS gateway might
receive and discard mitigation requests from the DOTS client,
ensuring no requested mitigation is ever applied.

To detect misuse, as detailed in Section 2.4, DOTS implementations
require mutual authentication of DOTS agents in order to make agent
impersonation more difficult. However, impersonation may still be
possible as a result of credential theft, implementation flaws, or
DOTS agents being compromised.

To detect compromised DOTS agents, DOTS operators should carefully
monitor and audit DOTS agents to detect misbehavior and deter misuse
while employing best current practices to secure network
communications to reduce attack surface.

Blocking communication between DOTS agents has the potential to
disrupt the core function of DOTS, which is to request mitigation of
active or expected DDoS attacks. The DOTS signal channel is expected
to operate over congested inbound links, and, as described in
Section 2.2, the signal channel protocol must be designed for minimal
data transfer to reduce the incidence of signal loss.